In printing using printing plates made with computer-to-plate (CTP) technology, overprinting is generally set for black characters (also referred to as “black ink characters”). In overprinting, when a black character overlaps with another figure that includes a color other than black, black ink is printed over ink of the other color in the area where the black character and the other figure overlap each other. This prevents white streaks from appearing around the black character during printing even if there is a slight shift in position relative to printing paper between a black printing plate and a printing plate of the other color. In printing using printing plates, black ink and the other color ink are superimposed one on another, so that black characters are represented in a darker tone (rich black) on printing paper.
Japanese Patent Application Laid-Open No. 2005-167970 discloses a technique in which, for example, when an overprint attribute of a figure in a portable document format (PDF) image file is “OP=false (knockout),” trapping is performed on the figure and a related lower-level figure. Thus, trapping is performed only when there is a risk of gaps appearing along the borders. Japanese Patent Application Laid-Open No. 2008-259101 discloses an image processor in which a received command group (image data) is interpreted to bitmap component images corresponding to a plurality of color materials, and attribute information included in the command group is used to generate attribute information of each pixel for each component image. Then, processing for changing the shape of objects is performed for each component image. For example, in the case where trapping is performed between a character portion of a cyan component image and a background portion of a magenta component image, the attributes of the trapped portions of the cyan component image and the magenta component image after the trapping are respectively a character attribute and a graphic attribute. Then, processing such as dithering is performed, using the attribute information and the image information for each component image. The above-described processing can improve image quality as compared with the case where each pixel at the same position has a common attribute for all component images.
Incidentally, when the overprint attribute of a black figure such as a black character is ON and the black figure overlaps with another figure that includes a color other than black in original image data before the original image is converted into raster data for image recording performed by an inkjet printer, black ink and the other color ink are ejected onto the area of printing paper where the black figure and the other figure overlap each other. If these inks are mixed in liquid form on the surface of the printing paper, the color of the black figure cannot be represented purely (i.e., the figure will be an impure black color) in the image recorded on the printing paper. It is conceivable to set the overprint attribute of each figure to OFF in the original image data, but if there is a shift in ink ejection positions between heads, white streaks will appear around the figure in the recorded image. In addition, the hues of figures other than black figures will be different from the intended hues.